- the first picture, the token has not been received, you have to upload the json key file to post the token and get the google token

- the second picture, the json key file was successfully uploaded and the token was successfully obtained, if you use the demo application, you can only upload 1 json key file to use, if you want to replace it you can press the replace button in the third picture

- in the third picture, the demo application will change the appearance of the file upload form to disappear and be replaced with information that an existing file is being used or uploaded, so it is forbidden to upload new files when the previous token is being used

when you want to use the service application, you can use the demo application to save the json key file, or you can create your own application to handle the server action